1. Field of the Invention
The invention relates to crosslinkable compositions based on organosilicon compounds, more particularly RTV-1 sealants, in which finely divided metal oxides with low surface area are used in combination with hydrocarbon mixtures, and which in the cured state have a high tear resistance, to methods for producing them, and to their use.
2. Background Art
One-component silicon rubber mixtures which are storable with exclusion of water but undergo condensation crosslinking to elastomers at room temperature on ingress of water (RTV-1 compositions) are long-established products. They are employed in large quantities in, for example, the construction industry, as a sealant for joints. The bases of these mixtures are polydiorganosiloxanes which include at least two reactive groups, such as OH, acyloxy, oxime, alkoxy, or vinyl groups, for instance, in the molecule. These compositions may further comprise fillers, plasticizers, crosslinkers, catalysts, and additives.
The performance properties of the cured product and also the specific properties of the crosslinkable composition during application are determined primarily by the average chain length of the crosslinkable polydiorganosiloxanes, by the presence of plasticizers, and by the activity of the finely divided reinforcing fillers, which correlates with, among other factors, the BET surface area of the filler particles. Furthermore, other components as well, such as adhesion promoters and crosslinkers, have an influence on certain paste properties and vulcanizate properties. The influence of catalysts on the mechanical vulcanizate properties of the composition can very often be disregarded.
The individual raw materials and their proportions in the RTV-1 composition are often selected so as to obtain an extremely balanced relationship between the physical properties. This is important especially since optimizing the properties of the compositions in the uncured state often undesirably alters the properties of the composition in the cured state. For example, it is indeed the case that a high proportion of reinforcing fillers, such as fumed silicon dioxide, in the RTV-1 sealant results in a desirably high “body”, i.e., in a relatively high viscosity and hence in a greater resistance to the deformation when the compositions are being smoothed in a joint that is to be sealed. At the same time, however, a greater proportion of reinforcing filler also reduces the rate of extrusion of the paste from the cartridge and drastically increases the modulus of the vulcanized composition (the value of the tensile stress at a defined strain of a test specimen), which is undesirable. This often then means that at least one of these properties cannot be set optimally for the application.
Plasticizers influence the rheological properties of the uncured paste and also the mechanical properties of the cured silicon rubber; for example, replacing silicone polymer by silicone plasticizers or organic plasticizers increases the extrusion rate, but the hardness is reduced.
Further important properties of the cured and uncured RTV-1 sealant that can in general not be influenced independently of one another are, for example, sag resistance, volume shrinkage, or else adhesion to different substrates.
Crosslinkable polydiorganosiloxanes used typically for RTV-1 sealants in the construction sector are high-viscosity polydimethylsiloxanes having hydrolyzable end groups and a dynamic viscosity of 10,000 to 350,000 mPa·s at 25° C. They crosslink by elimination of, for example, alcohols, oximes, or acetic acid. The plasticizers used are required to be highly compatible with the crosslinkable polydiorganosiloxanes, so that they remain in the product and are not excreted as an oily liquid from the vulcanized silicone rubber (a phenomenon known as vulcanizate bleeding, exudation, or oiling-out). Preferred plasticizers, accordingly, are either polydiorganosiloxanes without hydrolyzable groups or hydrocarbon mixtures. Customary polydiorganosiloxane-based plasticizers are trimethylsilyl-terminated polydimethylsiloxanes which have a dynamic viscosity in the range from 100 to 10,000 mPa·s at 25° C. Suitable organic plasticizers are described in EP-B1-885 921, for example, such as paraffinic hydrocarbon mixtures which have an initial boiling point above 290° C., a kinematic viscosity of 5 mm2/s to 10 mm2/s, as measured at 40° C., a viscosity-density constant (VDC) of less than 0.82, and a molecular weight of not more than 310 g/mol, and which contain 60% to 80% of paraffinic, 20% to 40% of naphthenic, and not more than 1% of aromatic carbon atoms. The disadvantage of these plasticizers, however, is that when used in substantial amounts of more than 40 parts by weight, based on 100 parts by weight of crosslinkable polydiorganosiloxane in the RTV-1 mixture, incompatibilities with the vulcanized siloxane matrix are manifested by exudation of constituents with relatively high molecular weight, especially if the mixture at the same time includes constituents which influence the rheological properties of the paste, such as polyglycols. Moreover, the vulcanizates identified in the working examples of EP-B1-885 921 have low tear resistance values; according to the present state of the art, construction sealants have tear resistance values of between 3 and 6 N/mm, as measured by DIN ISO 34-1 Method C. The latter also applies to RTV-1 mixtures which comprise hydrocarbon mixtures with a relatively low boiling range. Highly compatible hydrocarbon mixtures are known, for instance, from U.S. Pat. No. 6,451,440 B2, containing 45-75 wt % of naphthenic and 25-55 wt % of noncyclic paraffinic hydrocarbons. Other hydrocarbon mixtures which have very high compatibility with the high-viscosity, crosslinkable polydimethylsiloxanes lead frequently to an undesirably high level of volume shrinkage, exhibit incompatibilities with the siloxane matrix at relatively low temperatures, or lead rapidly to the yellowing of the material.
Fillers used are primarily finely divided, fumed silicon dioxides, since with these it is also possible to produce transparent compositions. For technical reasons, finely divided silicon dioxides having BET surface areas of 130 to 200 m2/g have become established. Grades with 130 to 150 m2/g are used with preference. EP-B1-1 884 541, for example, describes a method for continuous production of RTV-1 sealants, using a silicon dioxide having a BET surface area of 150 m2/g.
Finely divided, pyrogenic silicon dioxides with lower activity, i.e., having BET surface areas of below 120 m2/g, are used hardly at all in practice, since their reinforcing effect is inadequate. Where low quantities are used, compositions are obtained that have low viscosity and hence inadequate sag resistance, and, when large quantities are used, compositions are obtained that have an excessive modulus.
US-A1-2008/0245476 describes the use of silicon dioxides having BET surface areas of 10 to 90 m2/g for producing silane-crosslinking polyethers featuring high tensile shear strength. At a proportion of 15 wt % of finely divided silica, however, the plasticizer-free compositions described are unsuitable for sealing construction joints, owing to insufficient stretchability. Moreover, compositions based on polyethers have a chemical stability which is much too low for exterior applications.
Finely divided, fumed silicon dioxides having very high BET surface areas of more than 150 m2/g have too high a thickening effect. Moreover, these silicon dioxides likewise have the disadvantage that it is impossible simultaneously to reconcile performance properties such as excellent sag resistance, optimum extrusion rates, and low values for tensile stress under strain. Furthermore, compounding with these highly active silicon dioxide grades is substantially more difficult, since their dispersibility is significantly poorer, particularly at the necessarily relatively low amounts employed.
RTV-1 sealants which contain no other fillers such as calcium carbonate, for example, customarily comprise finely divided silicon dioxides in amounts of about 6 to 12 wt %, based on the total weight. The compositions are then easily expressed from cartridges, have sufficiently high resistance to deformation during smoothing, do not run out from vertical joints, and cure to rubber-elastic materials with a low modulus. Such compositions are therefore very well suited to the sealing of construction joints. An unsatisfactory feature, however, is the low tear resistance of these sealants. It would therefore be desirable if RTV-1 sealants with high tear resistance could be provided.